Injection and acceleration of energetic particles at collisionless shocks

While the theory of shock acceleration has been widely accepted in astrophysical settings, the source and injection mechanism of suprathermal particles remains a subject of ongoing discussion. We will summarize and compare different techniques, e.g. analytic theory, Monte Carlo simulations, in-situ observations and hybrid simulations, which have been used in the past to address this problem. Recent analytical studies and Monte Carlo simulation assume that the downstream heated plasma can leak through the shock into the upstream region and act as the source of energetic upstream ions. In contrast, from observations upstream of the Earth's bow shock it has been suggested that specularly reflected ions evolve into diffuse ions. In this paper we will present evidence that none of these processes is responsible for upstream energetic ions. By using hybrid simulation and following the trajectories of diffuse ions in phase space we found that many ions leave the shock already energized after their first shock encounter: they are trapped at the shock and gain energy when their tangential velocity is in phase with the tangential wave electric field. We discuss several numerical experiments which show that the seed particles for diffuse upstream ions are solar wind ions which are trapped at the shock, gain energy in the wave electric field, and are reflected back upstream.